The present invention relates to a method and apparatus for inducing a photochemical reaction upon irradiation of light, especially, a laser beam.
In a recent process for manufacturing semiconductor devices such as LSIs, extensive studies have been made on photochemical vapor deposition (photo CVD) and photoetching with a view to obtaining low-temperature and contamination free processes, and simplifying manufacturing steps. An ultraviolet light source capable of causing electron excitation has been mainly used as a light source for photochemical reactions induced by light in photo CVD and photoetching. Typical examples are excimer lasers (wavelengths of 157 nm, 193 nm, 248 nm, 308 nm, etc.), and a second harmonic (a wavelength of 257 nm) of an Ar laser (a wavelength of 515 nm). However, these conventional light sources present the following problems.
When an excimer laser is used, a pressurized toxic gas container for handling a toxic gas such as F.sub.2 or HC1, an exhaust gas processing unit, a unit for exhausting the gas into air, a leakage sensor, and various interlock mechanisms synchronized with the leakage sensor must be used. For this reason, the light source should have a large size and high cost and cannot be easily moved. In addition, high grade safety control is required. At present, therefore, the excimer laser cannot be used as a practical light source.
On the other hand, the second harmonic of the Ar laser has an output power of at best only several milliwatts. Even if a high power is obtained, the second harmonic continuously oscillates, so that a region of a photochemical reaction, induced by thermal diffusion on the substrate, becomes larger than the size of the beam spot, thereby failing to provide microprocessing on the order of micrometers. Although the second harmonic of the Ar laser does not have drawbacks caused by the use of toxic gases, it is an ultraviolet light source which is not suitable for photochemical reactions.
One of the candidates of practical ultraviolet light sources is exemplified in Japanese Patent Disclosure No. 57-67161 entitled "Laser Thin Film Formation Apparatus" whose embodiment describes an example using a fourth harmonic (a wavelength of 266 nm) of a Q switch Nd:YAG laser. In this description, Cr(CO).sub.6 is used. However, no specific reason is given for the selection of the light source. Nor is description made for the results of laser CVD.
When the fourth harmonic of the Q-switch Nd:YAG laser is actually used to induce a photochemical reaction, it is important to obtain a high-power fourth harmonic. In order to improve conversion efficiency of the harmonic generation, a flash-lamp excited Q-switch Nd:YAG laser with a high peak power is normally used. In this case, a possible high-power fourth harmonic has a repetition frequency of about 50 Hz, a peak power of about 100 kW and a pulse width of about 10 ns. Hence, the pulse width and the repetition frequency of the fourth harmonic are substantially the same as those of the excimer laser. In contrast, the peak power of the fourth harmonic is approximately one thousandth of that of the excimer laser. However, the peak power density per unit area of the fourth harmonic can be regarded as substantially the same as that of the excimer laser, in consideration of a difference between the sectional areas of the harmonic beam and the excimer laser beam. Therefore, when these beams irradiate corresponding substrates through identical optical systems, the characteristics of the fourth harmonic are substantially the same as those of the excimer laser. Under the above assumption, when the fourth harmonic is used to induce a photochemical reaction, the same results as in using the excimer laser are to be expected. In practice, the present inventors found that the same result as in using the excimer laser was obtained when Cr CVD was performed using Cr(CO).sub.6 gas. In this case, the drawbacks presented by use of the excimer laser can be eliminated. A fourth harmonic beam of the flash-lamp excited Q-switch Nd:YAG laser can provide the same photo CVD characteristics as the excimer laser can.
Even with the fourth harmonic of the flash-lamp excited Q-switch Nd:YAG laser having a high probability of practical application, the following problems occur in inducing the photochemical reactions.
Refractory metals such as tungsten (W), molybdenum (Mo) and chromium (Cr) are useful as a wiring or electrode material of LSIs. In laser CVD of Cr carbonyl Cr(CO).sub.6 is decomposed to deposit Cr on a substrate. In this case, Yokoyama et al. experimentally found (Conference on Laser and Electro-Optics Technical Digest, June 19-22, 1984) that a high quality metallic luster film could not be formed only by optical decomposition using a 50-Hz KrF excimer laser, but could be obtained when the deposited film was sufficiently heated simultaneously by light. This was also confirmed by the present inventors when the fourth harmonic of the flash-lamp excited YAG laser was used. According to another experiment of the present inventors using the same YAG laser, when the deposition area of Cr was decreased by decreasing the beam spot size down to less than several tens of micrometers, metallic luster of the deposited film was lost, thus degrading the quality of the film. In this case, when energy per pulse was increased so as to improve the heating effect, damage occurred in the film due to radiation, thereby failing to obtain a high-quality film.
According to reports for laser CVD of refractory metals, good films in small regions could not be obtained. For example, in a paper written by Solanki, Applied Physics Letter, Vol. 38, PP. 572-574, 1981, a film having a diameter of about 100 .mu.m is deposited using a copper ion laser (a wavelength of 260 to 270 nm) at a repetition frequency of 40 Hz. In this paper, no description is made for the quality of the deposited film, hence it is difficult to ascertain that a film of high quality was obtained. In a paper written by Solanki, Applied Physics Letter, Vol. 41, PP. 1048-1050, 1982, a film of high quality was obtained by using an excimer laser, having a repetition frequency of up to 300 Hz. However, the area of the deposited film was as large as several square centimeters and no attempt was made to deposit a film in a small area of the order of micrometers to several tens of micrometers (hereafter abbreviated as microarea).
As is apparent from these conventional problems concerning light sources, conventional methods and apparatuses for inducing a photochemical reaction cannot adequately form a high quality CVD film in a microarea.